1. Field of the Invention
The present invention relates to a flash-lamp-pumped or laser-diode-pumped solid-state laser amplifier with thermal birefringence compensation.
2. Related Background Art
A known, flash-lamp-pumped, passive four-pass Nd: glass solid-state laser amplifier is, for example, the one described in IEEE Journal of Quantum Electronics, Vol. 30, No. 4, pp. 884-886, 1994. Further, a method for compensating for thermal birefringence occurring in a laser diode (LD) pumped Nd: glass solid-state laser medium by using a quartz 90.degree. rotator is, for example, the one described in Review of Laser Engineering, Vol. 24, No. 3, pp. 343-352, 1996.
FIG. 2 is a schematic plan view to show the configuration of a passive four-pass solid-state laser amplifier with thermal birefringence compensation in the prior art. In this solid-state laser amplifier, solid-state laser media (which are normally of a solid-state laser material doped with active elements such as Nd, Yb, Ho, or Er, or the like) 7 and 8 optically pumped by light sources (pumping sources) 9 such as flash lamps or LDs have the same dimensions and are optically pumped in the same state. Owing to this, thermal birefringence is produced in the same distribution in the laser media 7 and 8.
The operation of the solid-state laser amplifier of FIG. 2 will be described. Source light 2 as p-polarized light (light polarized in the horizontal direction) from pulsed laser oscillator 1 passes through polarization beam splitter 3 without loss and is then incident to Faraday rotator 4. By this, the azimuth of the source light is rotated by 45.degree.. After this, the azimuth angle of the source light is further rotated 45.degree. by half-wave plate 20 in the opposite direction to the direction of rotation by the Faraday rotator 4, so that the source light becomes p-polarized light again. After that, the source light passes through polarization beam splitter 5 without loss.
The source light emerging from the polarization beam splitter 5 is incident to the optically pumped laser media 7 and 8 in order, thereby being amplified and being affected by the thermal birefringence in the laser media 7 and 8. However, since the laser media 7 and 8 have the same thermal birefringence distribution and since quartz 90.degree. rotator 10 is provided midway between the laser media 7 and 8, this quartz 90.degree. rotator 10 compensates for the thermal birefringence effect due to these laser media. Specifically describing, by the thermal birefringence effect of the laser medium 7, the linearly polarized light incident thereto is emerged as elliptically polarized light. Without the rotator 10, thus ellipitically polarized light incident to the laser medium 8 is emerged as much distortion light. In contrast, when the rotator 10 is provided, the thermal birefringence is compensated for, so that the light emerging from the laser medium 8 to the right is linearly s-polarized light (light polarized in the vertical direction). In this case, the thermal lens effect usually takes place in the optically pumped laser media 7 and 8, but the thermal lens effect can be neglected if the focal length of this thermal lens is sufficiently longer than the length of the laser amplifier. If the focal length of the thermal lens is short, the thermal lens effect can be compensated for readily by disposing a concave lens near 90.degree. rotator 10. Therefore, no problem is posed in compensating for the thermal birefringence by the 90.degree. rotator 10.
The amplified, pulsed laser light emerging as s-polarized light from the laser medium 8 to the right then passes through quarter-wave plate 11 to the right to become circularly polarized light. Then the laser light is reflected by total reflection mirror 12 to change the traveling direction to the left. This reflected laser light of circularly polarized light passes through the quarter-wave plate 11 to the left to become p-polarized light. This p-polarized light then passes through the optically pumped laser medium 8, the 90.degree. rotator 10, and the optically pumped laser medium 7 in order, thereby being amplified again. The thermal birefringence caused in the laser media is also compensated for here by the rotator 10, so that this laser light becomes s-polarized light to be emitted from the laser medium 7 to the left.
After this, the laser light is totally reflected by the polarization beam splitter 5 to be incident to total reflection mirror 6. It is reflected here and thereafter is incident again to the polarization beam splitter 5 to be reflected totally, thereby traveling to the right. Owing to this, this laser light of s-polarized light is subject to third optical amplification by the laser media 7 and 8 and the thermal birefringence is compensated for by the 90.degree. rotator 10. Thus, the laser light becomes p-polarized light when it is emitted from the laser medium 8 to the right. Next, this p-polarized laser light passes the quarter-wave plate 11 forward and backward through reflection at the total reflection mirror 12 to become s-polarized light and to be incident again to the laser medium 8. This laser light passes through the laser media 8 and 7 in order to the left to be subject to fourth optical amplification and compensation for the thermal birefringence by the 90.degree. rotator 10, thereby becoming p-polarized light to be emitted from the laser medium 7 to the left.
This amplified, p-polarized, pulsed laser light passes through the polarization beam splitter 5 without loss and then passes through the half-wave plate 20 to the left. Owing to this, the azimuth angle of the laser light is rotated 45.degree. to return to the original azimuth angle. However, the azimuth angle is further rotated 45.degree. during passage through the Faraday rotator 4 to the left, so that the azimuth angle is rotated 90.degree., turning the laser light to s-polarized light. This laser light is emitted from the Faraday rotator 4 to the left and is totally reflected by the polarization beam splitter 3. This obtains the pulsed laser output light 13 of linearly s-polarized light.
In the passive four-pass solid-state laser amplifier with thermal birefringence compensation of FIG. 2, however, optical power of the source light, i.e., the pulsed laser output of laser oscillator 1, must be considerably high in order to sufficiently extract energy accumulated in the laser media as achieving saturation amplification, and there are demands for realization of a solid-state laser amplifier that can achieve saturation amplification by lower power of the source light.
The present invention has been accomplished to meet such demands and an object of the present invention is to realize a solid-state laser amplifier that can achieve saturation laser amplification using output laser light from a pulsed laser oscillator of relatively low output, as the source light.